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Nakajima B6N Jill

In 1939 the Imperial
Japanese Navy drew up its specification for a carrier-based
torpedo-bomber to supersede the Nakajima B5N. The specifications issued
by the navy called for very modern characteristics. A maximum speed of
288 mph (463 km/h), a cruising speed of 230 mph (370 km/h) and a range
of 1,000 nautical miles (3335 km) without a bombload. To meet the
requirement, Nakajima decided to use an airframe very similar to that
of the earlier aircraft, differing primarily in its vertical tail
surfaces. The navy had specified use of the Mitsubishi Kasei radial
engine, but Nakajima decided to use instead its own 1,870 hp (1395 kW)
Nakajima NK7A Mamoru 11 radial engine of similar output driving a four
bladed Hamilton type propeller. The first of two prototypes was flown
in spring 1941, but initial flight testing revealed a number of
problems, including engine vibration and overheating, but the most
serious was that of directional stability, requiring revised vertical
tail surfaces. Final flight testing carried out aboard the aircraft
carriers Ryuho and Zuikaku in the end of 1942, revealed
further problems with the tuning of the engine and the need to
reinforce the arrester hook and landing gear. It was not until February
1943 that the type entered production as the Navy Carrier Attack Bomber
Tenzan Model 11, company designation Nakajima B6N1, incorporating a
number of refinements as a result of extended flight testing. However,
after only 135 production Tenzan (heavenly mountain) aircraft had been
delivered a new crisis arose when Nakajima was ordered to terminate
manufacture of the Mamoru engine, and use the more reliable 1,850 hp
(1380 kW) Mitsubishi MK4T Kasei 25 engine, a step also taken to allow
greater emphasis to be placed on production of the widely-used Nakajima
Homare and Sakae engines.

The company was now
compelled to use the engine which the navy had specified originally,
the Mitsubishi Kasei, but fortunately the adaptation of the B6N
airframe to accept this powerplant presented no major difficulties. The
resulting aircraft, which was also the major production version, had
the designation B6N2 and differed only from the B6N1 by the
installation of the Mitsubishi Kasei 25 engine. The B6N2a variant had
the rear-firing 7.7 mm (0.303 in) machine-gun replaced by one of 13 mm
(0.51 in) calibre. When production ended, Nakajima had built a total of
1,268 B6Ns of all versions, this number including two modified B6N2
airframes which had served as prototypes for a proposed land-based B6N3
Model 13. The powerplant had been the improved 1,850 hp (1380 kW)
Mitsubishi MK4T-C 25C version of the Kasei engine and the strengthened
landing gear had larger wheels for operation from unprepared runways,
but production did not start before the war ended. Allocated the Allied
codename 'Jill', the B6Ns saw intensive use during the last two years
of the war for conventional carrier operations and, in the latter
stages, in kamikaze roles.

Variants

Nakajima B6N2 -
Nakajima was ordered to cease using the Mamoru engine and use instead
the Mitsubishi Kasei 25 engine, thus resulting in the redesignated
B6N2. Although the Kasei 25 was slightly less powerful, this was offset
by introducing a less drag version of the exhaust ports which also gave
a slight jet-thrust like boost effect.

Nakajima B6N2a - This
type differed from the B6N2 only by having a rear firing machine gun of
13 mm (0.51 in) calibre, instead of of the 7.7 mm (0.303 in) type used
on the B6N2.

Nakajima B6N3 - Two
conversions of the B6N2a resulted in the B6N3 prototypes equipped with
1,850 hp (1380 kW) Mitsubishi MK4T-C Kasei 25C engines for evaluation
as land based bombers.